Method of dehydrating aqueous solutions of formic acid



Oct. 18, 1949.

H. M. GUINOT METHOD oF DEHYDRATING AQUEous SOLUTIONS 0F FORMIG ACID Filed June 15, 1945 CONDE NSE R 5E T7' LING SEPARATOR AZEOTRop/c a/sr/LLArloN COL IINN MM; www

Patented Oct. 18, 1949 METHOD OF DEHYDRATING AQUEOUS Y SOLUTIONS OF FORMIC ACID Henri Martin Guinot, Versailles, France, assigner to Les Usines De Melle (Societe Anonyme),

Saint-Legerles-Melle;-France, a company France Application June 15, 1945, Serial No. 599,606 In France April 4, 1941 Section 1, Public Law 690, August 8,1946

' Patent expires April 4, 1961 v 1o ciaims.- (ci. 2oz-4a) l It is known that azeotropic dehydration of Iormic acid solutions causes particular dlfliculties by reason of the fact that iormic acid forms with water an azeotrope having a maximum boiling point; the azeotrope boils at 107.1 C. and contains 77 parts of acid and 23 parts of water.

Hence, when an aqueous solution o-f formic ,acid

having an acid content above 77 per cent "is distilled, formic acid exhibits a tendency to .rise to the top of the column, thereby practically precentration is withdrawn from the apparatus and discarded after recovery through distillation of the slight amount of water entrainer dissolved therein; the other fraction is returned to the top venting the insoluble liquid employed as a water o entrainer to perform its function.

In U. S. Patent No. 1,896,100 dated February 7, 1933, there has been described a method capable of doing away with the diiilculty; according to said method, azeotropic distillation is carried out in the presence of a dilue-nt having a high `boiling point which may be the Water entrainer itself and is adapted to hold the acid in the bottom part of the column. Dehydration is performed in one step in the distilling column; a second column is used merely to separate anhydrous acid from diluent.

It is a primary object of my invention to provide a novel, simpler and more efficient manner of dehydratng aqueous solutions of formic acid.

According to this invention, aqueous formic acid to which no diluent has been added and which may have been brought, if desired, by distillation to the concentration corresponding to azeotrope, is introduced into the middle part of a column that has been charged once for all with a suitable amount of a water entrainer, said 'amount being so selected as to leave in the lower a minimum boiling point which is withdrawn from the top of the column and is caused to settle into two layers. The upper layer containing water entrainer is returned as a whole into the clumn. As to the lower layer consisting of water substantially free from formic acid, it is split into two portions; one of them corresponding to the necessary amount for securing desired conof the column together with the upper layer. The water return which is a characteristic feature of this invention has a twofold purpose:

On the one hand, togshove down to the foot of the column by washing effect, any formic acid which would tend to rise to the top;

On the other hand, to keep Within the column a sufficiently large amount of water for maintaining the water entrainer engaged in an azeotropio mixture in that part of the column where it is designed 'to operate and preventing the same to sink to the foot as it would have a tendency to do when its boiling point differs but little-from that of formic acid or is higher.

Owing to' the above feature, an aqueous layer free from formic acid is obtained in the decanter while formic acid' at about 85 to 95 per cent is Withdrawn substantially free from water entrainer from the foot of the column.

The partly dehydrated acid is then subjected to simple distillation in a second column from the top of which part of formic acid is obtained in perfectly anhydrous condition while the remainder is recovered from the foot as a 77 per cent azeotrope which is returned to the first column. As a water entrainer preference will be given to formic esters, particularly butyl formate (B. P. 106.9 C.), isobutyl formate (B. P. 97.9 C.), amyl formate (B. P. 130.4" C.) and isoamyl `formate (B. P. 123 C.). Nevertheless, it is possible without departing from the spirit of this invention to use other entrainers, for instance ethers such as dibutyl ether, ketones such as methylpropylketone, and so on.

Where acid to be dehydrated has a concentration substantially above 77 per cent, it may be fed directly to the second column, the rst column being still used for treating 77 per cent acid obtained as a tail in the second column.

Bearing in mind that acid concentration on the various plates regularly increases from the acid ingress point in the first column down to the foot and from the acid ingress point in the second column up to the top thereof, it will readily be understood that the method in accordance with this invention enables of treating acid of any concentration; sufce it to provide for each column a menaces plurality oi acid supply points and to introduce the acid to be treated at a point wherethe concentration oi' acid boiling on the opposite plate corresponds to that of the supply liquid. j

It is still within the spirit of this invention bring about minor modications in the above described method; for instance, instead oi vwithdrawing anhydrous formic acid from the very top of the second column, it may be advantageous to withdraw it laterally in so-called pasteurized form so as to reserve in the uppermost portion of the column some plates whereon come .slight amounts of water entrainer which in spite of the precautions taken might have reached the foot of the first column.

The said entrainer admixed with a large proportion of formic acid and possibly some amounts of water is withdrawn at the required speed and returned to the first column wherein its components classify themselves.

The following examples with reference to the diagrammatic appended drawing will better show the character of this invention.

Example 1.-Let us consider a 77.per cent solution of formic acid.

Column 3Vbeing charged once for all with a suitable amount of butyl formate (hatched portion); formic acid to be dehydrated is transferred from tank I into the middle portion of the column through pipes 2, 2a. Owing to butyl formate being presenten column plates, azeotrope waterbutyl formate boiling at 85 C. is readily formed. It is condensed in condenser I, then forwarded to decanter or settling separator wherein it settles into two layers; the upper layer is returned as a whole into the upper portion of column 3 through pipe 8; as to the lower layer, composed oi' water free from acid but containing a small amount of dissolved formate,Y it is partly withdrawn at the required speed through pipe l while the remainder is returned to the upper portion of column 3 through pipe 3.

In the foot portion of column 3 wherein a temperature of l03.5 C. is maintained, formic acid at a concentration of about 90 per cent gathers practically free from .butyl formate;.it flowsl by gravity into column I0 through pipe II. From the top portion oi the latter, anhydrous formic acid is laterally withdrawn through pipe I2 ywhile 77 per cent azeotrope boiling at 107 C.

and behaving as a tail is shoved down to the foot' wherefrom it flows into tank I3. Pump I4 forwards it from tank I3 into tank I through pipe I5.

Controllable means I8 are provided on the return pipe from the condenser to column I0 for withdrawing any slight amounts of butyl formate which might have entered column I0. Said formate, admixed with formic acid and possibly` water, is returned to column 3 through pipe I1 opposite acid ingress points.

Assuming that original acid has a concentra- -spectively will beused for acid supply to the prises continuously introducing aqueous formic acid to be dehydrated' into a cycle for formic acid including, spaced apart therein and placed \in intercommunication thereby, an azeotropic distillation column and a simple distillation column, the azeotropic distillation column containing a predetermined amount of a water entrainer which is adapted to form an azeotrope with water but no azeotrope'with formic acid and is stable in the presence of formic acid, said azeotropic distillation column being operated to remove part of the water from the aqueous formic acid, thereby leaving an aqueous formic acid having an acid content considerably above 77 per cent to be passed to the simple distillation column, while the latter is operated to separate anhydrous formic acid which rises to the top thereof. from residual 77 per cent acid which returns through said cycle to said azeotropic distillation column; continuously' discarding said water; and continuously withdrawing anhydrous formic acid from the cycle out of the-top portion of the simple distillation column.

2. A continuous process for dehydrating aqueous formic acid having an acid content not substantially less than 77 per cent, which comprises continuously introducing the aqueous acid to be dehydrated into a primary cycle for formicacid including, spaced apart therein and placed in intercommunication thereby, the lower portionvv of an azeotropic distillation column andthe lower portion of a simple distillation column, the azeotropic distillation column containing a predetermined amount of a water entrainer which is adapted to form an azeotrope with water but no tion slightly different from 77 per cent, it will be fed to column 3 either through pipe 2b or through pipe 2c according as said concentration is above are to be dehydrated, pipes lad, llc or Ila reazeotrope with formic acid and is stable in the presence of formic acid, said azeotropic distillation column being operated to produce entrainerwater azeotrope vapours as a head product and leave as a bottom product, an aqueous formic acid having an acid content considerably above 77 per cent which is introduced through said primary cycle into the simple distillation column above the bottom thereof, while said simple distillation column is operated to separate anhydrous formic acid, which rises to the top thereof, from .residual 77 per cent formic acid which is returned from v the bottom of said simple distillation column through said primary cycle to and above the bottom of said azeotropic distillation column; continuously withdrawing said azeotrope vapours from thetop of said azeotropic distillation column; condensing said vapours in a condensing zone to obtain an aqueous condensate: causing said condensate to settle into two layers comprising an aqueous layer, in a decanting zone; discarding part of said aqueous layer, while refiuxing the remainder of said condensate to said top portion of said azeotropic column so that said water entrainer is continuously cycled through a secondary cycle including in sequence said top portion of said azeotropic distillation column,` said condensing zone and said decanting zone; and continuously withdrawing anhydrous formic acid from said primary cycle. out of the top portion of said simple distillation column.

assenso 3. The process oi claim 2, the aqueous formic acid to be dehydrated being introduced into the primary cycle at a point thereof where a formic acid concentration substantially equal to the concentration of said aqueous formic acid prevails.

4. The process of claim 2, the predetermined amount of water entrainer in the azeotropic distillation column and the amount of water removed from the decanting zone being such that the total amount of water entrainer in the azetropic distillation column is engaged as au azeotrope with water in said column.

5. The process of claim 2, the predetermined amount of water entrainer in the azeotropic distillation column being such that the water entrainer in said column is wholly confined above said point of egress of the aqueous formic acid.

6. The process of claim 2, anhydrous formic acid being withdrawn from the top portion of the simple distillation column at a point below the uppermost point of said column.

7. The process of claim 2, anhydrous formic acid being withdrawn from the top portion of the simple distillation column at a point below the uppermost point of said column, the process further comprising the step of withdrawing vapours from said simple distillation column at a point above the withdrawal point for anhydrous formic acid; condensing said vapours into a liquid, returning part of said liquid into the top portion of said simple distillation column and the remainder into the azeotropic distillation column. f

8. The process of claim 2, the water entrainer being selected from the group consisting of aliphatic formic esters having and 6 carbon atoms, dibutyl ether and methyl propylketone.

9. A continuous process for the production of anhydrous formic acid, which comprises cycling aqueous formic acid successively through the lower portion of an azeotropic distillation column and the lower portion of a simple distillation column, the azeotropic distillation column containing a predetermined amount of a water entrainer which is adapted to form an azeotrope with water but no azeotrope with formic acid and is stable in the presence of formic acid, said azeotropic distillation column being operated to produce entrainenwater azeotrope vapours as a head product and to leave as a bottom product an aqueous formic acid having an acid content considerably above 77 per cent which is intro duced through said cycle into the simple distillation column above the bottom thereof, while said simple distillation column is operated to separate anhydrous formic acid which rises to the top thereof, from residual 77 per cent formic acid which is returned from the bottom of said simple distillation column through said cycle to and above the bottom of said azeotropic distillation column; withdrawing said azeotrope vapours from the top of said azeotroplcdistillation column; condensing said vapours to obtain an aqueous condensate; causing said condensate to settie into two layers comprising an aqueous layer; discardu ing a portion of said aqueous layer; renaming the remainder of said condensate to the upper por tion of said azeotropic distillation column; with"A drawing anhydrous formic acid from the top por tion of the simple distillation column; and feeding aqueous formic acid having a strength above 77% to said simple distillation column at a level where formic acid in said column has substantially the same strength as the aqueous formic acid thus fed.

10. A continuous process for the production of anhydrous formic acid, which comprises cycling aqueous formic acid successively through the lower portion of an azeotropic distillation column and the lower portion of a simple distillation column, the azeotropic distillation column containing a predetermined amount of a water en trainer which is adapted to form an azeotrope with water but no azeotrope with formic acid and is stable in the presence of formic acid, said azeotropic distillation column being operated to produce entrainer-water azeotrope vapours as a head product and to leave as a bottom product an aqueous formic acid having an acid content considerably .above 77 per cent which is introduced through said cycle into the simple distillation column above the bottom thereof, while said simple distillation column is operated to separate anhydrous formic acid which rises to the top thereof, fromresidual 77 per cent formic acid which is returned from the bottom of said simple distillation column through said cycle to and above the bottom of said azeotropic distillation column; withdrawing said azeotrope vapours from the top of said azeotropic distillation column; condensing said vapours to obtain an aqueous condensate; causing said condensate to settle into two layers comprising an aqueous layer; discard ing a portion of said aqueous layer; reiluxing the remainder of said condensate 'to the upper portion of said azeotropic distillation column; with drawing anhydrous formic acid from the top portion of the simple distillation column and feeding aqueous formic acid having a strength oi about 77% to said azeotropic distillation column at a level where aqueous formic acid therein has substantially the same strength as the acid thus fed.

HENRI MARTIN GUINOT.

REFERENCES @MED The following references are of record in the iile of this patent:

UNITED STATES PATENTS Number Name Date 1,826,302 Clarke et al Oct. 6, 1931 1,896,100 Ricard et al. Feb. 7, 1933 2,049,441 Gordon Apr. 4, 1936 2,107,527 Evans Feb. 8, 1938 2,160,064 Eversole May 30, 1939 

